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Comment: 25:25 - 26:50 (01:25)
Source: Annenberg/CPB Resources - Earth Revealed - 18. Metamorphic Rocks
Keywords: "James Sadd", metamorphism, crust, "Earth's surface", "metamorphic rock", "oceanic crust", "metamorphic grade", fossil, life, "geologic time", "plate tectonics", mountain, erosion, ocean, continent, temperature, pressure, collision, "convergent plate margin", subduction, melting, magma

Our transcription: Metamorphism is a fundamental rock forming process on Earth.
About 15 percent of all continental crust exposed at the surface is composed of metamorphic rocks, and much of the oceanic crust is metamorphosed to a low grade as it formed.
Just as fossils are a record of life through time, metamorphic rocks are used to study the history of the Earth.
They allow us to reconstruct the movement of plates that no longer exist and to study mountain ranges that have long since worn away.
Like the opening of new oceans, the movement of continents, and the creation of mountain ranges, metamorphism is a consequence of plate tectonics.
The rise in temperature and pressure that makes metamorphism possible is almost always linked to plate movement and mountain building.
The collisions, the intrusions, and fault zones that metamorphose the rocks are concentrated at plate margin.
As rocks are depressed to great depth, say tens of kilometers in a subduction zone or placed under the great compression of a continental collision, metamorphic conditions can become so intense that the rocks begin to melt.
The magma rises buoyantly toward the surface setting the stage for the formation of new rocks and new metamorphic transformations.
When we study metamorphic rocks, we're seeing a brief glimpse of this cycle of rock formation and change, a cycle that's as old as the Earth itself.

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